Light olefins, such as ethylene, propylene and butylenes, can be produced using various processes such as steam cracking, fluid catalytic cracking, conversion of methanol to olefins, paraffin dehydrogenation, alcohol dehydration, methane coupling and Fischer Tropsch reactions. However, these processes often produce varying levels of acetylenic and/or diene by-products, such as acetylene, methyl acetylene (MA), propadiene (PD, butyne and butadiene. These by-products must be removed from the light olefin streams because they can act as poisons to the downstream processing catalysts, such as polymerization catalysts. The preferred method of removing these by-products is by selective hydrogenation in which, for example, acetylene is converted to ethylene, methyl acetylene and propadiene are converted to propylene, and butyne and butadiene are converted to butylenes.
Currently, the commercial catalysts used for this selective hydrogenation comprise nickel or palladium, such as palladium and silver, on an alumina support. However, in addition to producing the desired olefin products, these catalysts tend to generate significant quantities of saturates (for example, ethane, propane and butanes) as a result of over-hydrogenation and green oil (olefin oligomers) as a result of competing oligomerization reactions. Both of these by-products are undesirable in that they reduce the selectivity to the required light olefins. However, the green oil is particularly problematic in that it decreases the life of the hydrogenation catalyst.
There is therefore a need for an improved catalyst for the selective hydrogenation of alkynes and diolefins, wherein the catalyst exhibits increased olefin selectivity and reduced selectivity to saturates and oligomers, such as green oil, while retaining high hydrogenation activity.
U.S. Patent Application Publication No. 2002/0068843 discloses a catalyst for selectively hydrogenating acetylenic and diolefinic compounds with low green oil formation, the catalyst comprising the following active components loaded on a porous inorganic support: (1) at least one of platinum, palladium, nickel, ruthenium, cobalt, and rhodium; (2) at least one of silver, copper, zinc, potassium, sodium, magnesium, calcium, beryllium, tin, lead, strontium, barium, radium, iron, manganese, zirconium, molybdenum, and germanium; (3) at least one rare earth metal selected from scandium, yttrium, and Lanthanides in Group IIIB of Periodic Table of Elements; and (4) bismuth. Preferably, component (1) is platinum or palladium; component (2) is silver, potassium, or sodium; and component (3) is lanthanum or neodymium.
U.S. Pat. No. 6,255,548 discloses a method for selectively hydrogenating a feed comprising an acetylenic compound and/or a diolefin in the presence of a catalyst comprising at least one support, at least one Group VIII metal selected from nickel, palladium, platinum, rhodium, ruthenium and iridium and at least one additional element M selected from germanium, tin, lead, rhenium, gallium, indium, thallium, gold, and silver, wherein the catalyst is formed by introducing said additional element M into an aqueous solvent in the form of at least one water-soluble organometallic compound comprising at least one carbon-M bond. The preferred Group VIII metals are nickel, palladium and platinum and the preferred additional elements M are germanium, tin, gold, and silver.
U.S. Pat. Nos. 5,356,851 and 5,364,998 disclose a catalyst and a process for the selective hydrogenation of unsaturated compounds, wherein the catalyst contains 0.1 to 10% of at least one Group VIII metal selected from nickel, palladium, platinum, rhodium and ruthernium and 0.01 to 10% of at least one Group IIIA metal selected from gallium and indium, the metals being deposited on a support. The preferred Group VIII metals are nickel, palladium and platinum.
Co-pending U.S. patent application Ser. No. 10/720,617, filed Nov. 24, 2003, describes a catalyst and process for selectively hydrogenating alkynes and/or diolefins, wherein the catalyst comprises support on which is deposited (a) a rhodium component present in an amount such that the catalyst composition comprises less than 3.0% of rhodium by weight of the total catalyst composition; and (b) an indium component present in an amount such that the catalyst composition comprises at least 0.4% and less than 5.0% of indium by weight of the total catalyst composition.
Co-pending U.S. patent application Ser. No. 10/720,558, filed Nov. 24, 2003, describes a catalyst and process for selectively hydrogenating alkynes and/or diolefins, wherein the catalyst comprises a support, at least two different metal components selected from Groups 8 to 10 of the Periodic Table of Elements, and at least one metal component selected from Group 13 of the Periodic Table of Elements. The metal components can be added to the support by impregnation or co-precipitation.
In addition to the types of metal used in the catalyst, the properties of supported catalysts such as those described above may vary significantly depending on the distribution of the metal(s) in the catalysts. For example, the metal(s) may be substantially uniformly distributed throughout the support, can be located within a thin layer at the support surface (commonly referred to as eggshell), can be located at the center of the support (commonly referred to as egg yolk), or can be concentrated between the outer edge and the center of the support (commonly referred to as egg white).
Thus, U.S. Pat. No. 3,859,377 discloses a catalyst for the selective hydrogenation of butadiene that comprises 0.01 to weight % palladium which is at least partially-through or deep-impregnated to a depth of at least 0.12 inch on a kieselguhr support.
In contrast, U.S. Pat. No. 6,096,933 discloses a supported hydrogenation catalyst composition which comprises a palladium component, at least one alkali metal iodide such as, for example, potassium iodide, and an inorganic support material such as alumina, wherein the palladium component is concentrated in an area within about 150 microns of the exterior surface of the composition.
In addition, a supported rhodium catalyst is disclosed in U.S. Pat. No. 4,420,420 in which active rhodium metal is provided on a silica type or titania type support such that the rhodium is present within a depth of about 0.4 mm of the surface of the support. The catalyst optionally contains one or more co-catalysts including alkaline earth metals, such as calcium, magnesium, barium and the like, noble metals, such as platinum, palladium, iridium, ruthenium, gold and the like, iron, nickel, cobalt, cerium and manganese.
Further, U.S. Pat. No. 6,586,647 discloses a catalyst for selectively hydrogenating C4-acetylenes in a liquid hydrocarbon stream containing largely butadiene wherein the catalyst comprises an inorganic oxide support having dispersed thereon finely divided copper metal and an activator metal selected from nickel, cobalt, platinum, palladium, manganese, and a combination thereof where the copper metal and activator metal are dispersed on the support using impregnation where the volume of the impregnating solution is less than that required to fill the pore volume resulting in at least 50 weight % of the copper metal and the activator metal being dispersed on the outer 200 micron layer of the support.